Miniature resonant photonic devices as known in the art are created from coupled high Q-factor cavities, for example, ring or photonic crystal resonators. The resonance is a result of circulating whispering gallery modes (WGMs) that are created within circular structures as an optical signal travels around the circumference of the structure, undergoing repeated internal reflections at near-grazing incidence. The leakage of light can be very small in these structures, leading to high intrinsic quality factors (Q factors). The Q factor is generally defined as a measure of energy loss relative to the energy stored in a resonator (or any type of oscillating device), characterized by the center frequency of a resonance divided by its bandwidth. The preferred “high Q” resonator is therefore associated with a relatively narrow and sharp-peaked resonance feature.
These miniature resonant photonic devices may be fabricated into more complex structures which can be coupled to each other and perform more complex functions (structures such as, for example, filters, dispersion compensators, delay lines, and the like), in a manner similar to conventional planar photonic resonant microstructures.
Conventional resonator structures are formed by creating features whose size is of the order of the wavelength of the propagating optical signal, or greater. For example, known rings or toroids or spheres are typically tens of microns in dimension. Such structures are commonly created using lithographic techniques (for example, etching a silicon material to create the feature pattern) with the undesirable result of surface roughness. The lithography-associated roughness leads to scattering of a propagating optical signal, reducing the Q factor of the device. In addition, the conventional fabrication process inaccuracies limit the precision with which multiple devices can be coupled together to form more complex structures. While it would be useful to create resonator structures with even smaller dimensions (i.e., sub-wavelength), which offers certain advantages in terms of performance, such smaller dimensions pose additional difficulties in fabrication. Thus, the required accuracy of sub-micron fabrication is difficult to achieve and the necessary techniques for achieving reproducible results of the accuracy required for optical devices have not yet been developed.